UV Cured finishes - The differences in materials

UV CURED FINISHES

THE DIFFERENCES IN MATERIALS

by Brian Howard luthier

Magnolia, De.

  One thing you will find when you start working with these materials is how different they can be. Unlike urethane finishes which have some minor differences in buffabilty and flow between brands these materials can be vastly different in almost every regard when comparing materials from different manufacturers. For the purposes of this discussion we will be comparing clear gloss topcoats developed specifically for guitars from Hentzen Aerospace PST-908 and CureUV (formerly SPDI) Wetlook Topcoat. Both are UV cure only, that and the fact they are high gloss and water white are about the only similarities between the two. When buffed out there is no visual distinction that can be made between these choices though I expect some slightly different wear characteristics down the road.

 Differences can be noticed right out of the can. The Hentzen coating is more viscous. As such the Hentzen needs some MEK to make sprayable. The CureUV is thinner and needs no reduction, in fact adding MEK will make it too thin to use. The Cure UV is a true 100% solids while the Hentzen is around 80% as sprayed. This makes for big differences in how these materials are sprayed right down to the gun and set up each prefers.

The Hentzen material likes a bigger needle and tip, around 1.5mm. Even after reduction to spray the Hentzen material is still very thixotropic relying on the shearing at the air cap to cause a viscosity drop to facilitate short term flow and then gel. As such it has very good hang and can be sprayed easily at 3-4mils wet per coat and double coated between curing sessions. It should be pointed out here that the Hentzen also needs more wet coats for the same final build as it is not quite 100% solids.

The CureUV will be happier with a .08-1.0mm. needle and tip. I run a slightly bigger fan pattern with it, 6" wide instead of my typical 4". This material is low in viscosity! As such it does not have the vertical hang of the other material and can only be applied at about 2 mils per wet coat. There is no real thixotropic action and it does not gel on the surface, so wet stacking coats between curing sessions cannot be done here. Every sprayed coat must be completely cured before another can be applied. These differences mean a few other things at this point. The CureUV has much less surface tension as well which really decreases the build at all the edges which is a problem with most polyester materials. As such this material is less prone to those little "finish resistant" pockets that happen with heavier materials of this type.

Curing between the two is a little different. Due to the solvents present in the Hentzen system you must wait 15-20 minutes for the coating to flash before curing or solvent pop may be an issue. The CureUV can be cured as soon as it has finished flowing out. This can mean less surface contamination as these materials at times tend to exhibit a static attraction for dust particles which can cause those little "finish Resistant" spots. The Hentzen material is overall a bit easier to cure. The CureUV seems to need more energy so will take some extra time or wattage. The CureUV also does not cure well when the coating is applied to thin so there is a narrow window of coating weight between very difficult to cure and sags & runs.

Sanding both inter-coat and prior to buffing is also different. The CureUV material is harder to sand. It dulls and clogs paper much faster than the Hentzen material. The CureUV requires a more complete scuff sand to get a good bond, all gloss must be removed. Whereas the Hentzn will tolerate a more standard scuff between coats with small bits of gloss peeking. The extra difficulty in sanding here will be discouraging but the Cure UV makes up for that in the buffing and in the amount of prep sanding needed.

Buffing these two finishes is very very different. They both share the high resistance to buff through, especially at the edges. Both like to be buffed fairly hard and anything other than buffing with solid wax on an arbor is not recommended for these type finishes. Prep sanding  the Hentzen product to buff requires sanding up through the grits to at least 800, preferably 1000. It is very difficult to see the sanding on the surface here and if it is not complete you will need to stop buffing and sand some more working through a few grits. Buffing then commences with Menzerna P204 or equivalent. The CureUV product buffs much easier. So easy in fact I can do something I haven't been able to do since switching to polyester finishes. Level sand once at 400 and then start buffing with Menzerna 113GZ. This adds an extra buffing step but saves much sanding! Any hidden surface texture that does appear after the pre-polish can be addressed with 600 and moved right to the P204 compound. 

Overall the CureUV materials;

• More forgiving of surface flaws in substrate
• Harder to cure
• Harder to sand
• Easier to buff
• May require an extra coat/curing session

Hentzen materials;

• Easy sanding
• Easy to cure
• Difficult to buff
• Can be made dual cure
• Must wait for solvent flash before curing

So as you can see unlike other types of finish systems where differences between urethanes by brand and system are usually pretty minor with these systems they can be almost unrecognizable as the same thing based on their application behavior. I used to recommend staying in a brand/system to eliminate compatibility issues. I now recommend sticking with a particular product so you have an always repeatable finish schedule without always trying  to re-engineer the process. Both of these products are excellent choices for instrument finishes and in the end look identical. Choosing the one that best fits your shop and work style is the key to success. So if you started down this road to what I think is the best finish yet and are disappointed in the results, don't give up. Try a different material or get some hands on training such as I offer here at my shop. 

For UV finishing classes and workshops please feel free to contact me.

Brian Howard luthier

Howard Guitars

Magnolia, DE 19962

(302) 335-2035